Direct acting solenoid actuators are often used to control fluid pressure in a variety of systems, including clutch mechanisms and other devices in an automobile. Direct acting solenoid actuators employ an armature mechanism that drives a fluid control element, such as a spool, a spring-biased four-way proportional flow control valve, a poppet valve and the like in various hydraulic control applications. Typically, the armature is connected to, and drives, a push pin that engages the fluid control element to this end.
A change in the electrical current supplied to the solenoid results in a change in fluid pressure. Ideally, a given input current corresponds to a single pressure, independent of whether the input current is increasing or decreasing. For example, consider a solenoid that is initially at high pressure (20 bars) at zero current. When a 0.5 Amp current is applied, the pressure drops to 12 bars. Ideally, if the current is increased to 1 Amp, and then decreased back down to 0.5 Amps, the pressure will again be 12 bars. Thus, a pressure value can be determined for each value of the current, independent of whether the current is increasing or decreasing.
In reality, a number of factors contribute to hysteresis in solenoid actuators. Hysteresis describes the difference in output for a given input when the input is increasing versus decreasing. In a direct acting solenoid actuator fluid control valve, friction between the armature and the armature sleeve, or between the spool and the nozzle body surrounding the spool, may prevent the armature and spool from sliding smoothly in response to the induced magnetic field. This may result in different values of pressure for a given current, depending on whether the current is increasing or decreasing. As such, the reliability of the fluid control valve decreases, and the direction of the current (increasing or decreasing) must be taken into account when selecting a current for achieving a desired pressure.
In an effort to improve the reliability of the fluid control valve, the spool and nozzle body may be machined such that the spool fits tightly within the nozzle body, but is still able to move axially in response to the force of the induced magnetic field on the armature. This machining, however, requires a high level of precision. Further, any variation in the spool design may require a corresponding change in the nozzle body.
Thus, there is a need for direct acting solenoid actuators that reduce or minimize hysteresis during operation without requiring additional high-precision machining.